Technique of using crosslinked foamed products to obtain resins having low specific gravity, i.e., lightweight properties, softness and high mechanical strength is widely used for building interior or exterior trim materials, automobile parts such as interior trim and door glass run channels, packaging materials, daily necessities, etc. The reason is that bonding of molecular chains through crosslinking reaction of a resin makes it possible to attain lightweight properties of the resin by foaming with inhibiting lowering of mechanical strength, though only foaming of the resin to obtain lightweight properties induces lowering of mechanical strength of the resin.
Also for footwear and footwear parts such as soles (mainly midsoles) of sports shoes, crosslinked foamed products of resins are employed, and the reason is that materials which are lightweight, inhibit deformation caused by long-term use and have such mechanical strength and impact resilience that they can withstand severe use conditions are desired.
For the shoe soles, a crosslinked foamed product of an ethylene/vinyl acetate copolymer has been heretofore used and is widely known. The crosslinked foamed product obtained by the use of the ethylene/vinyl acetate copolymer has a problem that it has high specific gravity and high compression set, so that when it is used for shoe soles, the shoe soles are heavy, and if the shoe soles are used for a long period of time, they are compressed and lose mechanical strength such as impact resilience.
In National Publication of International Patent No. 501447/1997 and Japanese Patent Laid-Open Publication No. 206406/1999, an invention relating to a crosslinked foamed product using an ethylene/α-olefin copolymer and an invention relating to a crosslinked foamed product using a mixture of an ethylene/vinyl acetate copolymer and an ethylene/α-olefin copolymer are described, respectively. By the inventions, however, satisfactory performance has not been obtained though the low specific gravity and the property of the compression set have been improved.
In Japanese Patent Laid-Open Publication No. 344924/2000 filed by the present inventors, an olefin elastomer crosslinked foamed product, which has high expansion ratio, is free from surface roughening due to defoaming and has soft touch, low compression set, excellent tear strength properties and excellent heat resistance, and an elastomer composition for the crosslinked foamed product are disclosed. That is to say, this crosslinked foamed product is obtained by heat-treating an olefin elastomer composition comprising an ethylene/α-olefin copolymer (A) having a density of 0.88 to 0.92 g/cm3 and a melt flow rate (190° C.) of 0.1 to 10 g/10 min, an organic peroxide (B), a crosslinking assistant (C) and a blowing agent (D) The ethylene/α-olefin copolymer (A) comprises 5 to 95 parts by weight of an ethylene/α-olefin copolymer (A1′) having a density of not less than 0.88 g/cm3 and less than 0.90 g/cm3 and a melt flow rate (190° C.) of 0.1 to 50 g/10 min and 5 to 95 parts by weight of an ethylene/α-olefin copolymer (A2′) having a density of 0.90 to 0.93 g/cm3 and a melt flow rate (190° C.) of 0.1 to 50 g/10 min, the total of said components (A1′) and (A2′) being 100 parts by weight, and the mixture of the ethylene/α-olefin copolymers (A1′) and (A2′) has a melt flow rate (190° C.) of 0.5 to 10 g/10 min. In this invention, the ethylene/α-olefin copolymer (A2′) having a relatively high density is incorporated in order to secure a hardness necessary for the resulting crosslinked foamed product, for example, a hardness necessary for footwear. The hardness necessary for footwear cannot be secured by only incorporation of the ethylene/α-olefin copolymer (A1′) of a relatively low density alone, and the resulting crosslinked foamed product is unsatisfactory for footwear.
The ethylene/α-olefin copolymer (A2′), however, has a problem. That is, in order to increase a surface hardness of the resulting crosslinked foamed product, the copolymer (A2′) is liable to have a higher melting point as compared with the ethylene/α-olefin copolymer (A1′), and hence kneading operation becomes difficult. More specifically, the copolymer (A2′) has a problem that in the kneading process, a high energy is necessary or mixing is not made well, and in an extreme case, a kneading device is broken. In case of a usual composition containing a high-melting point component, the above-mentioned kneading problem is solved by elevating the kneading temperature. In case of a foamable composition, however, the kneading operation needs to be carried out at a temperature lower than the prescribed temperature (usually not higher than 100° C.) because the elevated kneading temperature decomposes the blowing agent and/or the crosslinking agent.
The present inventors have earnestly studied to solve the kneading problem mentioned above, and as a result, have found that the kneading can be facilitated and a crosslinked foamed product having a hardness enough for a footwear foamed product can be obtained by adopting an ethylene/α-olefin copolymer (A2) having a density of 0.900 to 0.930 g/cm3, a melt flow rate (ASTM D 1238, 190° C., load of 2.16 kg) of 0.1 to 50 g/10 min and a main melting point peak at a position of not higher than 110° C., instead of the above-mentioned ethylene/α-olefin copolymer (A2′) and that an excellent interlaminar adhesive strength of the laminate comprising a layer of the foamed product and a layer of at least one material selected from the group consisting of a polyolefin, a polyurethane, a rubber, a leather and an artificial leather can be obtained especially when both the above-mentioned ethylene/α-olefin copolymers (A1) and (A2) contained in the composition of the foamed product are ethylene/1-butene copolymers. Based on the finding, the present invention has been accomplished.